Mozilla's security chief, Window Snyder, said the organization used test URLs provided by PhishTank through their public XML feed of valid phishing URLs. Snyder said her organization chose PhishTank's URLs in order to provide the fairest test comparison between the two browsers.

"That was our data source for the URLs in the test, so they were completely independent from the list of URLs we use for our blocked URL lists," she said. "We wanted to make sure it was as fair as possible."

The security team used 1,000 URLS to compare each browser's effectiveness with each brand's best anti-phishing filtering turned on. According to Mozilla, Firefox 2 was able to block 81.5 percent of the URLs, while Microsoft's IE7 with Auto-Check On was only able to block 66.4 percent.